This invention relates to the field of separation of liquids using stripping means. The process of the invention involves vapor stripping followed by mechanical compression of the vapor which is then exposed to a permeation membrane for separation of the compressed vapor.
Distillation is the most common separation unit operation for miscible liquid mixtures in the chemical industry, including facilities producing ethanol. Although distillation has proven to be a robust and efficient method for removing and concentrating ethanol (EtOH) from fermentation broths, the energy efficiency of distillation declines dramatically below an ethanol concentration of 5 wt %. Processes proposed for the conversion of lignocellulosic biomass and waste materials to ethanol may deliver ethanol concentrations less than 5 wt %. As a result, processes which recover ethanol from water more efficiently than distillation at low concentrations will make lignocellulosic ethanol more economically viable. Further, standard distillation is only able to reach the ethanol-water azeotrope—about 5 wt % water. To reach fuel-grade water levels (<1.3 wt % water), distillation is typically followed by molecular sieve dryers. A process which could efficiently produce fuel grade ethanol from dilute ethanol would be of great interest. Such a process would also be useful for recovering ethanol from dilute process or waste streams not currently attractive due to the inefficiency of distillation, particularly in smaller installations in which the economies of scale do not favor distillation. Such a technology would also be useful for the separation of other organic solvents from water and separation of organic solvent mixtures.
Gas stripping, shown schematically in FIG. 1a (prior art), has been proposed as a method for recovering volatile products from fermentation broth. The ability of an inert gas to remove these products under mild temperature and pressure conditions is attractive. Unfortunately, the inert gas dilutes the volatile product, making recovery of the product by condensation more energy intensive. When the inert gas is replaced by only water vapor, as depicted in FIG. 1b (prior art), the process is called steam stripping. Although steam stripping is usually associated with high temperatures, operating the stripping column at reduced pressures enables operation at lower temperatures.
Both gas and steam stripping offer high degrees of separation when the vapor-liquid equilibrium (VLE) provides a strong concentrating effect or if the overhead condensate separates into two phases due to solubility limits of the components. However, in situations where the components are fully miscible and the VLE behavior is not highly favorable, stripping and overall separation efficiencies decrease. Such is the case for the separation of lower alcohols, such as ethanol, methanol, and propanol(s), from water. Due to the low partial pressure of the lower alcohols in a vapor phase in equilibrium with an alcohol-water solution (i.e. low activity), the volume of gas or steam required to strip a given mass of the alcohol is higher than for more volatile/less soluble compounds. In addition, lower alcohols are fully miscible with water in the overhead condensate. Finally, several of the lower alcohols form azeotropes with water, complicating the separation of the components in a VLE-based system.
Alternative technologies must be compared to the benchmark technology for the recovery of alcohols from water—distillation. According to Hawley's Condensed Chemical Dictionary (14th Ed.), “distillation” is defined as: “A separation process in which a liquid is converted to vapor and the vapor then condensed to a liquid. The latter is referred to as the distillate, while the liquid material being vaporized is the charge or distilland. Distillation is thus a combination of evaporation, or vaporization, and condensation.” Hawley's further defines “continuous distillation” as: “Distillation in which a feed, usually of nearly constant composition, is supplied continuously to a fractionating column, and the product is continuously withdrawn at the top, the bottom, and sometimes at the intermediate points.” According to Perry's Chemical Engineers' Handbook (7th Ed.) the fractionating column in distillation can be considered as being composed of two sections: “If the feed is introduced at one point along the column shell, the column is divided into an upper section, which is often called the rectifying section, and a lower section, which is often referred to as the stripping section.” The stripping vapor for the stripping section is generated in a reboiler which may be indirectly heated with steam or with a combusted fuel. Alternatively, a vapor, such as steam, may be directly introduced to the column to generate the stripping vapor.
The stripping section of the column acts to remove the more volatile compounds from the falling liquid so that the liquid exiting the bottom of the column (the “bottoms” stream) is depleted in those compounds which preferentially partition into the vapor phase. The rectifying section acts to deplete the rising vapor of the less volatile species, thereby enriching the rising vapor in the more volatile compounds. Thus, in distillation columns a portion of the rising vapor at the top of the column is condensed and returned to the column to cause rectification/enriching of the more volatile species. The returned condensate is called “reflux”. At the bottom of the distillation column, a portion of the falling liquid is evaporated in the “reboiler” to create rising vapor. The reflux rate and the reboil rate are controlled to yield the desired operating conditions and compositions of the overhead and bottom product streams. The term “reflux ratio” is defined either as the ratio of liquid flow to vapor flow within the column (internal reflux ratio) or as the ratio of distillate returned to the column to the distillate withdrawn as product (external reflux ratio).
In conventional distillation columns, heat is added in the reboiler and removed in the overhead condenser. Although it acts to improve the separation performance of most distillation columns, return of reflux condensate to the column increases the heat required in the reboiler. In energy terms, the reboiler and overhead condenser are antagonistic. Thus, the reboiler heat required to perform a separation using a distillation column is greater than that required to simply strip the volatile species from the liquid.
The energy used to recover a unit mass of alcohol by distillation is a direct function of the feed concentration and reflux rate required to meet the desired product purity. As the concentration of alcohol decreases in the feed stream, the reflux rate increases and the amount of energy required to recover a unit mass of alcohol increases dramatically. This is shown graphically in FIG. 2 (Prior art) for the ethanol-water system as a gray area which represents a range of data provided in the literature for distillation processes. Also shown in FIG. 2 (prior art), as a horizontal line, is the heat of combustion of ethanol, about 30 MJ/kg. As indicated in the figure, the energy required to recover ethanol by distillation is low relative to the heat of combustion when the feed concentration is greater than 3 wt % ethanol. However, below 3 wt %, the energy of distillation rises rapidly and will exceed the heat of combustion when the feed concentration drops below 1 wt %. For this reason, the recovery of ethanol from streams containing less than 3 wt % is not very attractive from an energy standpoint.
In standard corn-to-ethanol production facilities, three separation columns (two stripping columns, one rectification column) combined with a molecular sieve dehydration system are used to recover ethanol from the fermentation broth and dry it to meet fuel specifications. Separate stripping and rectification columns are used instead of a single distillation column to improve heat integration, utilizing lower grade steam sources from within the recovery stage and from other unit operations in the facility. However, heat integration of this kind is more complicated to design/operate and requires additional equipment, thus increasing the capital cost. Such complexity is acceptable for larger scale operations, but becomes less so when the operation is scaled down and the relative cost increases. The beer still column, rectifier column, and side stripper column contain 22, 30, and 16 trays, respectively. Kwiatkowski et al. (J. R. Kwiatkowski, A. J. McAloon, F. Taylor, and D. B. Johnston, Modeling the process and costs of fuel ethanol production by the corn dry-grind process, Industrial Crops and Products, 23 (May 2006) 288-96) modeled the distillation system columns in a corn-to-ethanol facility with 34, 28, and 27 stages, respectively. Steam usage in those three columns treating 10.8 wt % ethanol broth totaled 25,146 kg/hr for an energy usage of 4.7 MJ/kg-ethanol (assuming 80% efficient boilers and an energy value of 2.26 MJ/kg-steam).
Prior art involving hybrid distillation-vapor permeation systems for alcohol-water separations rely upon rectification with condensed overhead vapor reflux liquid. The vapor feed to the vapor permeation system is either re-evaporated condensate or uncondensed overhead vapor. In all cases, however, a reflux condenser is an integral component of the system. Similarly, in hybrid distillation-pervaporation systems, the feed to the pervaporation system is condensed overhead vapor. Hybrid processes combining gas stripping with vapor permeation taught in prior art use non-condensable gases to remove volatile compounds from a liquid mixture. Other related prior art uses membranes to recover and recycle water vapor for the purpose of stripping non-condensable gases dissolved in liquids.
U.S. patent application Ser. No. 10/546,686 Ikeda (pub. no. US 2006/0070867 A1) “Method for concentrating water-soluble organic material”. Ikeda '686 teaches a method for concentrating water-soluble organic material combining distillation with vapor permeation and includes heat recovery from either or both the permeated and non-permeated vapor either directly or indirectly in reboiler. Ikeda '686 teaches the use of a distillation column with complete condensation of the overhead vapor and a return of a portion of that condensed overhead as reflux to the rectification section of the column. No overhead compressor is taught therein.
U.S. Pat. No. 4,978,430 by Nakagawa et al. “Method for dehydration and concentration of aqueous solution containing organic compound”. Nakagawa '430 teaches a combination of an “evaporation vessel”, in which heat is provided but no reflux is involved, with a water-selective vapor permeation membrane system. The process of Nakagawa '430 relies upon the temperature of the evaporation vessel with an optional “adjusting valve” to set the feed pressure to the vapor permeation system. Thus, the membrane feed pressure for Nakagawa '430 is coupled to the evaporation temperature. No overhead compressor is included. The process of Nakagawa adds heat to the vapor prior to the membrane system and requires cooling to produce condensed permeate.
U.S. Pat. No. 5,273,572 Baker (1993) “Process for removing an organic compound from water”. Baker '572 teaches the separation of organic compounds from water by gas stripping with organic compound removal from the gas using organic-selective membranes. Stripping gas may be recycled. Stripping gas may be water vapor i.e. “steam”, but the steam is at least partially condensed before the stream, contacts the membrane unit. Overhead from stripper may be compressed. Stripper may operate at reduced pressure. However, the invention of Baker does not produce dry solvent.
U.S. Pat. No. 7,070,694 by Coiling et al., “Purification of fluid compounds utilizing a distillation-membrane separation process”. Colling '694 teaches the combination of a distillation column, requiring reflux liquid for rectification with vapor permeation system for hydrocarbon purification. Coiling '694 teaches the use of a compressor on the vapor overhead from the column to raise the pressure of the vapor feed to the vapor permeation membrane and to enable recovery of latent heat from the overhead vapor by condensation of a portion of that overhead in the reboiler heat exchanger.
Sommer and Melin (2004) (S. Sommer and T. Melin, Design and optimization of hybrid separation processes for the dehydration of 2-propanol and other organics, Industrial & Engineering Chemistry Research, 43 (2004) 5248-59) discusses distillation-vapor permeation and distillation-pervaporation hybrids, all have reflux (“a pervaporation unit should be operated in such a way that the amount being separated by the membrane is as small as possible and withdrawn [from the distillation column] on the highest concentration level”). This article teaches against the presently disclosed invention.
Material published by Vaperma Inc. of St-Romuald, Quebec, Canada on their website www.vaperma.com shows a flow diagram of a process for producing ethanol in which overhead from a beer still is treated by membrane separation. No compression of the overhead stream from the still is shown, and the condensed permeate stream is returned to the fermentor, not the beer still. A presentation by Pierre Côté et al. at the International Fuel Ethanol Workshop in St. Louis, Mo. on Jun. 23, 2007, entitled Field Demonstration of the Sifiek™ Membrane for Ethanol Dewatering, and available subsequently on www.vaperma.com, shows a two-step membrane separation unit treating an ethanol/water mixture to create a dry ethanol product. The membrane separation steps operate under a driving force provided by a partial vacuum on the permeate side of the membranes.
U.S. Pat. No. 4,444,571 by Matson, “Energy-efficient process for the stripping of gases from liquids”. Matson '571 teaches an energy-efficient process for the removal of a noncondensable or high vapor pressure gas (such as carbon dioxide or ammonia) from a liquid, such as water, which combines a stripping process with vapor permeation membrane system. The gas is separated from the vapor leaving the stripper by the membrane unit, enabling recovery of latent heat by return of the condensable vapor directly to the stripping column or by condensation in a reboiler heat exchanger. Matson '571 teaches the desorption of dissolved gases, either from water or organic solvents. It does not teach separation of water-organic solvent mixtures. In all of the claims in Matson '571, the membrane is “substantially permeable” to the condensable vapor while “substantially impermeable” to the noncondensable gas. The process of Matson '571 would not be appropriate for the separations to be performed with the present invention because both the permeate and retentate streams of the present invention contain condensable vapors while only one of the streams in Matson '571 is condensable. Matson '571 teaches that the gas-free permeate vapor is much more economically compressed from an energy standpoint than is the overhead from stripping column stating that “this method [compressing the entire overhead mixture] is impractical because of the large energy requirement associated with compressing the stripped gas present with the vapor”. Thus, Matson '571 creates the membrane mass transfer driving force using a vacuum compressor only on the permeate stream which also enables recovery of the latent heat from the condensable permeate. In Matson '571, the membrane feed pressure is dictated by the temperature of the stripping column. The maximum pressure difference across the membrane is determined by the stripper pressure. Thus, the minimum membrane area according to the invention of Matson '571 can only be reduced by increasing the temperature of the stripper.